I'm not at all sure I understand this "far-field" performance note. In
Gary Reese's tests he always used a 1:40 magnification ratio. For a
17mm lens that's only 715mm or about 28" subject distance. Not far. At
17mm and full frame, even at f/2.8 the depth of field is still quite
considerable. It ranges from 584 to 923mm or 131mm (5.2") in front of
the subject to 208mm (8.2") behind the subject. That's a huge range of
DOF saving grace for field curvature. But I don't know. Maybe it
really is that bad.
And I really don't give much credence to user reports. For example, one
respondent says he uses the 17-35 on his 25.6MP Sony A900: "...i have
included an example showing the vignetting and strong corner softening
at 17mm wide open, but also results showing that f22 can be used on this
25.6 megapixel sensor for maximum depth of field without losing too much
sharpness by diffraction..."
Are we to believe this? Referencing the theoretical resolution of a
lens at f/22 against a 5D sensor yields a maximum of 4MP due to
diffraction. So, he thinks that taking his 25.6MP sensor down to 4MP is
"without losing too much sharpness"? That simply says that this owner
is not critical about lens resolution. It doesn't mean it won't make a
nice web image or even a good 8x10 print but it's not going to be 30x40
material.
But if you're happy with your results I certainly can't argue. I don't
own one. I do like its predecessor the 20-40/2.7-3.5. I don't know how
it really performs but seems to do well at f/11 which is where I usually
shoot it. I used it heavily during my trip. But I lost a lot of shots
since it flares like crazy if it gets anywhere near the sun. But I
can't complain for $70. :-)
Chuck Norcutt
Moose wrote:
> Chuck Norcutt wrote:
>> And for a contrary opinion for full frame on a 5D
>> <http://www.slrgear.com/reviews/showproduct.php/product/48/cat/23>
>>
>
> Sheesh! I looked at that site last night, didn't see the FF tab and went on.
>
>> Awful edge performance until you get to f/8, f/11. At f/11 it gives
>> consistent good (but not great) performance across the full zoom range. For
>> Canon shooters SLR Gear recommends the Canon 17-40 over the Tamron
>> 17-35. <http://www.slrgear.com/reviews/showproduct.php/product/31/cat/11>
>>
>
> Interesting stuff. The 6 user reviews of the Tamron average 9.0 for IQ,
> the 36 for the Canon average 8.92. At FM, 25 reviews average 8.8 for
> Tamron and 431 average 9.0 for the Canon. It appears the huge
> differences in the test results don't correlate with user experience.
>
> One factor may be:
> "/Far-field performance (Especially for wide-angle lenses)
> /Given the finite size of our studio and the targets available to us,
> the camera is always relatively close to the test target. (Especially
> with wide-angle lenses.) Good lenses generally have behave similarly at
> medium and long focal distances, but "far field" performance could be
> different for some models. - Pay attention to the reports and sample
> photos offered by other readers for this information."
>
> At 17mm, the subject to lens distance must be very short.
>
> I've just browsed through some of my images @ 17mm or so with this lens
> No definitive conclusions, but several observations.
>
> - I don't have very many images suitable for evaluating far zone
> sharpness, especially upper corners. In most shots, either there is no
> detail in the corners or it is OOF anyway from DOF.
>
> - Edge softness isn't much problem even @ f2.8, and gone by f4. The more
> correct term for the area of concern would be corner softness.
>
> - The left upper corner of mine is softer than the other three. The
> three are good way out almost to the deep corner. The other can be
> pretty fuzzy a bit further in, depending.
>
> - I say 'depending' because it appears the corner softness may be
> largely due to field curvature. So the results depend in part on the
> 'shape' of the subject. Most of the shots I took in Hearst Castle had
> edges closer to the camera than the center, and I think the field
> curvature worked in my favor.
>
> - Field curvature may vary quite a bit with focal distance. Just a hint
> from too few images to be sure.
>
> - I lost many of the far upper corners to linear lens and perspective
> distortion correction, so they don't figure into final images at all.
>
> I may look at this some more, but I have to say I'm still impressed with
> the lens overall. I will try to remember to stop down more where the
> light allows. But honestly, something over 99% of image area and images
> are nicely sharp.
>
> Moose
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